Conventionally, switching power supply units using semiconductor apparatuses for controlling switching power supplies are widely used as power supplies for home appliances such as home electrical products, thereby to improve power efficiency with low power consumption. In the semiconductor apparatus, an output voltage is controlled (stabilized) by using the switching operation of a semiconductor (a switching element such as a transistor).
Particularly in recent years, in view of the prevention of global warming, attention has focused on a reduction of power consumption in a standby state of appliances such as home electrical products and thus switching power supply units achieving lower power consumption during standby are in great demand.
In order to meet the demand, a power supply system or the like is developed for properly using two switching power supply units according to an operation mode of an appliance. For example, a switching power supply unit for a main power supply is provided to supply power at rated load in a normal operation (normal mode) of the appliance, and a switching power supply unit only for a standby state is separately provided to supply power during standby in a standby operation (standby mode) of the appliance. When the appliance is in a standby state, power is supplied from the switching power supply only for a standby state. At rated load, power is supplied from the switching power supply for the main power supply.
However, this power supply system requires two switching power supply units (converters), increasing the cost of the overall circuit including the semiconductor apparatuses for controlling the switching power supplies. Therefore, when lower cost is strongly demanded, a power supply system constituted of a single switching power supply unit (converter) is used in many cases. In this case, switching power supply units of partial resonance have been frequently used in view of efficiency of a power supply and noise.
In such a conventional semiconductor apparatus for controlling a switching power supply, a current applied to a switching element is reduced at a light load, e.g., during standby. However, it is always necessary to supply, via a transformer, the internal circuit current of the semiconductor apparatus for controlling the switching power supply. Therefore, it is not possible to reduce a current flowing to the switching power supply, including a current flowing to the switching element, to 0 and thus a certain amount of current is applied even at no load. Hence, the switching operation of the switching element causes a loss even at no load and a lighter load causes a larger loss in the switching element. Consequently, the switching power supply decreases in power efficiency and thus it is not possible to meet the need for lower power consumption in a standby state of the power supply.
Further, the switching power supply of partial resonance mentioned above has the following problem: since the switching power supply has a high oscillation frequency at a light load, a switching loss increases and efficiency decreases in a standby mode.
As a solution for reduced efficiency in a standby mode, a method is available for intermittently performing a switching operation in response to a standby signal from the outside. However, this method increases the cost because another expensive oscillation circuit has to be separately provided to intermittently oscillate a power supply in a standby mode. Further, in the event of an improper period and duty for intermittent oscillation, an output voltage has a higher ripple, resulting in no practical applicability.
Japanese Patent Laid-Open No. 2002-315333 discloses another conventional control technique, in which a microcomputer detects a loaded state on the secondary side of a power supply, a transition is made to a standby mode in response to a signal, and oscillation is performed intermittently according to a commercial frequency. However, an oscillation frequency cannot be changed according to the loaded state in an intermittent switching operation and thus the efficiency of the power supply during standby is not sufficiently improved.